The preparation of high quality semiconductor materials, that is, materials having a high degree of crystal perfection and desired compositional variations, is important in present day technology, and a variety of techniques has been developed to prepare such materials for use in devices such as integrated circuits, photodetectors, injection lasers, microwave oscillators, etc. For example, liquid phase epitaxy (LPE), chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) have been used to prepare device quality semiconductor materials.
The last named technique is very successful and has been an important approach in the successful effort to grow high quality Al.sub.x Ga.sub.1-x As, x being greater than or equal to 0.0 and less than or equal to 1.0, multilayer structures for use in applications such as optoelectronic and microwave devices. It is believed by some workers in the art that MBE will become the preferred crystal growth technology for the Group III-V Al.sub.x Ga.sub.1-x As materials system.
This optimistic assessment regarding the future of MBE is based on several factors and recent important advances. For example, large area, approximately 7.5 cm diameter, highly uniform AlGaAs double-heterostructure wafers which yielded laser diodes having both very low current threshold and good lifetime characteristics have been grown. Additionally, development of both ohmic contact and Schottky barrier structures as well as metallizations have been accomplished. Furthermore, certain types of devices are presently most expeditiously fabricated by MBE. These devices include high mobility modulation-doped semiconductor superlattices for field effect transistors, superlattices comprising ultra-thin alternating GaAs and AlAs layers, other structures having ultra-thin layers, and AlGaAs multilayers which have been grown at accelerated growth rates of approximately 12 .mu.m/h. All of these factors and advances suggest that MBE has great promise as a high through-put, high yield, and a highly reproducible method of fabricating AlGaAs multilayer structures for optoelectronic and microwave devices as well as integrated circuits. Successful work has also been done growing devices having other Group III-V, such as AlGaAsSb and InGaAs, layers by MBE.
It is now well known that the Group III-V quaternary InGaAsP materials system is important for optical fiber communications operating in the wavelength range between 1.0 and 1.65 .mu.m where the present silica-based glass fibers have low loss and dispersion. The InGaAsP materials, which are typically grown lattice-matched to an InP substrate, are used for both light sources, such as lasers and light emitting diodes, and photodetectors. This materials system may also find applications in microwave electronics and integrated optoelectronics. For microwave electronics, the InGaAsP materials have higher peak electron drift velocities than GaAs and Si, and also have better surface characteristics than GaAs. Such properties are important and desirable in both FETs and transfer electron devices. Additionally, a combination of the photonic and electronic devices fabricated with InGaAsP on the same InP substrate could well be the ultimate accomplishment of integrated optoelectronics.
At present, however, these Group III-V InGaAsP quaternary materials systems are prepared almost exclusively by liquid phase epitaxy because these materials have not yet been grown successfully by MBE due to several difficulties associated with the growth of phosphorus-containing compounds. For example, phosphorus has a very high vapor pressure and is highly reactive with several metals, such as copper, that are commonly used in MBE apparatus as sealing gaskets. These and other difficulties make the regular ultra-high vacuum MBE systems used for growing the AlGaAs and the other materials previously mentioned unsuitable for use with InGaAsP for at least the following reasons.
For example, conventional MBE systems are evacuated by ion pumps that are relatively inefficient in pumping phosphorus. When the phosphorus pressure is greater than approximately 10.sup.-5 torr for an extended period of time, the ion pumps tend to become overloaded and to cease pumping. The difficulties involved in pumping phosphorus become even greater when phosphorus-containing compound semiconductors are being grown because the sticking coefficient of phosphorus on the substrate or epitaxial layer surface is relatively small. In fact, the sticking coefficient of phosphorus is even smaller than the relatively small sticking coefficient of As. Consequently, the phosphorus pressure within the growth chamber is typically much higher than the arsenic pressure commonly present within the growth chamber during growth of, for example, AlGaAs. The high phosphorus pressure unavoidably either overloads the ion pump or prevents the growth of epitaxial layers at high growth rates.
Extensive cryo-panels can be used to condense the background phosphorus and hence can help in keeping the background phosphorus pressure low during growth. However, this approach also suffers drawbacks. First, at the end of the growth run and during the subsequent warming of the cryo-panels, the phosphorus will reevaporate and the resulting increase in the phosphorus pressure in the chamber will overload the ion pump. Moreover, this problem is not easily avoided because other types of pumps cannot be easily substituted for ion pumps in MBE apparatus. The use of ion pumps is important for the growth of high quality epitaxial layers, because, unlike other commonly used pumps, for example, oil diffusion pumps, there are no hydrocarbon contaminants from the ion pumps in the vacuum chamber. Additionally, the use of ion pumps is desirable because such pumps are highly reliable and require virtually no maintenance. Second, when phosphorus condenses on the cool chamber walls or cryo-panels, it is in the form of white phosphorus. When this material is exposed to air, it is extremely likely that it will catch fire and form P.sub.2 O.sub.5. This compound is hydroscopic and will then absorb water and form a gummy substance. This gummy substance contains phosphoric acid and will have deleterious effects on the MBE system. Consequently, if the phosphorus is not removed from the chamber before the chamber is exposed to air, it will not only contaminate the system with P.sub.2 O.sub.5 but will also create a potentially dangerous situation.